Current star formation in early-type galaxies and the K+A phenomenon

We present the results of an effort to identify and study a sample of the likely progenitors of elliptical (E) and lenticular (S0) K+A galaxies. To achieve this, we have searched a sample ~11000 nearby (m_r < 16) early-type galaxies selected by morphology from the Sloan Digital Sky Survey (SDSS) main spectroscopic sample for actively star-forming E and S0 galaxies. Using emission-line ratios and visual inspection of SDSS g-band images, we have identified 335 galaxies from the SDSS Fourth Data Release (DR4) as actively star-forming E and S0 galaxies. These galaxies make up about 3 per cent of the total early-type sample and less than 1 per cent of all main galaxies with m_r < 16. We also identified a sample of ~400 K+A galaxies from DR4 with m_r < 16; more than half of these are E and S0 galaxies. We find that star-forming early-type galaxies and K+A galaxies have similar mass distributions; they are on average less massive than typical early-type galaxies but more massive than the average star-forming galaxy. Both of these types of galaxies are found in higher fractions among all galaxies in lower density environments. The fractions of star-forming E and S0 galaxies and E and S0 K+A galaxies depend on environment in nearly the same way. Model spectra fit to the stellar continua of the star-forming E and S0 galaxies showed that their properties are consistent with star formation episodes of <~1Gyr in duration. The modelling results imply that on average, the star formation episodes will increase the stellar masses by about 4 per cent. The results also imply that only episodes that increase the stellar mass by more than 2-5 per cent will lead to K+A galaxies as we have defined them and that this is true for roughly 80 per cent of the star-forming E and S0 galaxies in our sample. The estimated typical increase in stellar mass implies that new stellar components of about 2 × 10⁸M_solar will be formed on average. There is also evidence that the star-forming regions within these galaxies are rotationally supported. These two results, when combined with the galaxies' total masses and lack of prominent discs, suggest that the star formation within these galaxies may be confined to relatively small, central discs, similar to the nuclear stellar and dust discs found in many low-mass early-type galaxies.